1. Field of the Invention
The present invention relates to a pneumatic tire comprising a tread surface including a land portion in which a sipe extending in a width direction of the tire is formed, more particularly to a pneumatic tire useful as a winter tire.
2. Description of the Related Art
Conventionally, winter tires have a land portion such as a block and a rib formed with pluralities of cuts called as sipes therein. These sipes generate edge effect and water removal effect and increase running performance of a tire on an icy road surface, which has a small friction coefficient (hereinafter, referred to as “ice performance”). As such sipes, straight sipes extending straight along a longitudinal direction thereof, wave-like sipes extending in a wave-like shape, or the like are practically used.
For example, Japanese Patent Application Laid-Open Publication No. 2007-223493 described below discloses a pneumatic tire as described below. That is, the pneumatic tire includes pluralities of blocks formed on a tread surface thereof, and each of the blocks is formed with one or more sipes extending in a width direction of the tire. In an opening portion, at the front side as viewed in a rotation direction of the sipe, an inclined plane, which is inclined in a rotation direction toward the outer side in the radius direction of the tire, is formed. In the above pneumatic tire, pluralities of inclined planes are formed at the opening side of the sipes. The edge portion of the inclined plane increases the edge effect and enhances the braking performance on an icy road surface (hereinafter, referred to as “ice braking performance”). However, the above pneumatic tire has no edge element acting in the width direction of the tire (turning directions). Therefore, the pneumatic tire still has a room to improve the turning performance on an icy road surface (hereinafter, referred to “ice turning performance”). Moreover, in such pneumatic tire, when the sipe is formed with an inclined plane of a larger inclination angle in order to increase the edge effect, the block formed with the sipes is reduced in the rigidity thereof. As a result, the steering stability on dry surface (hereinafter, referred to as “dry steering stability performance”) decreases, or the blocks tend to wear unevenly in the front and rear portions thereof.
Japanese Patent Application Laid-Open Publication No. 9-272312 described below discloses a pneumatic tire as described below. That is, the pneumatic tire includes pluralities of blocks formed on a tread surface thereof, and each of the blocks is formed with sipes extending in the width direction of the tire. Each of the sipes is arranged so that the width of the sipe is made wider than the other portion from the tread surface to the sipe bottom at least at three points including the both ends of the sipe. However, the pneumatic tire has no inclined plane at the opening side of the sipe. Therefore, the edge effect of the sipe is insufficient and the tire has a point to be further improved in both of ice braking performance and ice turning performance. Moreover, the sipe is arranged so that the width thereof is made wider than the other portion from the tread surface to the sipe bottom at least at three points including the both ends thereof. Therefore, the block formed with the sipe tends to lose the rigidity and dry steering stability performance is easily reduced; or the blocks tend to be worn unevenly in the front and rear portions thereof.
Japanese Patent Application Laid-Open Publication No. 2000-318413 described below discloses a pneumatic tire as described below. That is, the pneumatic tire includes a tread surface formed with pluralities of sipes thereon. The sipe includes a main portion extending in the identical direction and a connecting portion extending in a direction substantially perpendicular to the main portion. The neighboring main portions are arranged to be inclined in an opposite directions with respect to the radius direction of the tire so that the distance therebetween becomes larger as the depth increases. Japanese Patent Application Laid-Open Publication No. 11-7843 described below discloses a pneumatic tire as described below. That is, the pneumatic tire has a tread surface formed with pluralities of sipes thereon. The sipes are arranged to be separated away from each other at the tread surface which comes in contact with the road surface and become closer or connected to each other at the groove bottom side. However, these pneumatic tires fail in obtaining sufficient edge effect by the sipes. Therefore, the tires have a room to be improved in the both aspects of the ice braking performance and the ice turning performance.